Stephen Fry - He was like pretty much the first celeb on Twitter and IMO talks a lot of agreeable sense
Alan Sugar & Piers Morgan - Pretty rubbish to follow on their own, but entertaining for how much they bitch at each other.
Wil Wheaton - Because Star Trek TNG, and him being just as "interested" in his time on it as his fans (by contrast I actually un-followed Brent Spiner as I get the strong impression he wants nothing to do with either).
Tara Strong - She's Rikku and Twilight Sparkle, seems to love what she does and takes an interest in the characters she plays, and she's absolutely fantastic to her fans. What's not to love.
Dara O Briain - He's funny.
The Queen (@Queen_UK) (not the real one, but funny)
John de Lancie - He's Q, Discord and pretty cool.
Robert Llewellyn - Kryten off Red Dwarf, or the guy off Scrapheap Challenge and Carpool. One of the early Twitterers. Keeps us updated on the progress of things like a certain new Red Dwarf series that starts tonight~